In general, it is known that amorphous Ni-P (written as "a-Ni-P" in the following) has high mechanical strength and anticorrosion and is a non-magnetic material. Since a-Ni-P has those characteristics, a-Ni-P is used as a reinforcement and an anticorrosive film for machine parts, metallic tanks and the like by plating a-Ni-P on a surface thereof.
In particular, a-Ni-P is coated on an aluminium disc to use as a reinforcement for a substrate of a magnetic hard disc.
A substrate of a magnetic hard disc is made of plating a-Ni-P on a surface of an aluminium disc to form a coat layer made of a-Ni-P thereon, and a magnetic hard disc is a magnetic recording element which is made of forming a ferromagnetic material layer on a surface of the coat layer by sputtering and the like.
It is not easy to fix ferromagnetic materials over a surface of such a coat layer made of a-Ni-P (written as "an a-Ni-P coat layer" in the following). Accordingly, in the art, irregularities are formed on the surface of the a-Ni-P coat layer so that an area where the ferromagnetic material can be fixed over can be larger or the whole of the substrate which has been formed the a-Ni-P coat layer on an aluminium disc is heated so that the ferromagnetic material can be easily fixed thereon.
However, in the case that the irregularities are formed on the surface of the a-Ni-P coat layer, when reading magnetic records through a magnetic head, signals detected in dales of the irregularity are weak and the magnetic head may be collided with hills thereof, so that the magnetic recording density may not be improved.
Also, in the case that the whole of the a-Ni-P coat layer formed on the substrate is heated, Ni and Ni.sub.x Py crystals are produced over the whole of the a-Ni-P coat layer. Such Ni and Ni.sub.x P.sub.y crystals produced over the whole of the coat layer are too low in the mechanical strength to use as a reinforcement. (Where, the term "Ni.sub.x P.sub.y crystals" represents crystalized compounds of Ni and P, and subscripts x and y represent atomic composite ratios, respectively.)